vp9_rd.c quiet -Wstringop-overflow

vp9_rd.c quiet -Wstringop-overflow

../libvpx/vp9/encoder/vp9_rd.c:594:20: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
  594 |         t_above[i] = !!*(const uint32_t *)&above[i];
      |         ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../libvpx/vp9/encoder/vp9_rd.c:572:47: note: at offset [64, 254] into destination object ‘t_above’ of size [0, 16]
  572 |                               ENTROPY_CONTEXT t_above[16],
      |                               ~~~~~~~~~~~~~~~~^~~~~~~~~~~

Change-Id: Ie9ef24e685af417cdd35f6aa7284805e422b6ae2